1. Field of the Invention
The present invention relates to elastomers generally and, more particularly, but not by way of limitation, to a novel process for producing a thermoplastic elastomer having improved physical properties.
2. Background Art
Previously known processes to produce thermoplastic elastomers have involved dynamically vulcanizing a mixture of polyolefin and high molecular weight EPDM rubber. Such processes are described, for example, in U.S. Pat. No. 4,130,535, issued Dec. 19, 1978, to Coran et al, and U.S. Pat. No. 4,311,628, issued Jan. 19, 1982, to Abdou-Sabet et al. The general process is to mix the polyolefin and EPDM rubber at temperatures sufficient to soften or melt the polyolefin and then to add a curative which is preferably a phenolic curing resin A cure accelerator is added and mixing is continued until curing is complete. Mixing equipment employed may be any conventional masticating equipment, such as Banbury mixers, Brabender mixers, or certain mixing extruders. The thermoplastic elastomers thus produced exhibit relatively good oil resistance and physical properties compared to partially vulcanized and sulfur or peroxide cured compositions.
Although the above processes produce improved thermoplastic elastomers, the physical properties thereof are still somewhat deficient, due, primarily to the poor distribution of phenolic curing resin during the mixing/curing step. Distribution of phenolic curing resin in the blends of EPDM rubbers and polyolefin resin during dynamic vulcanization affects the final properties. Therefore, thorough dispersal of the curing resin before the cure accelerator is added is necessary to have highly desirable characteristics. However, since the phenolic resins have low melting temperatures and very low viscosities after melting, they are difficult to distribute into the highly viscous rubber matrix, in the short time available, to prevent any unwanted reaction such as self-polymerization. Accordingly, an improved method of distributing phenolic curing resin in the blends is needed to improve elastic properties as well as improved mechanical properties.
Accordingly, it is a principal object of the present invention to provide a method for producing thermoplastic elastomers which provides for improved distribution of phenolic curing resin in mixtures of polyolefin and high molecular weight EPDM rubber.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description.